Use is made for insulating industrial furnaces, as soon as the continuous operating temperature approaches or exceeds 1300.degree. C., of modular components made of refractory ceramic fibers of the "module" type, such as those described in French Patent No. 2,403,317 or its American equivalent U.S. Pat. No. 4,238,257, or of the "block" type, such as described in French Patent No. 2,507,594 or its American equivalent No. 4,440,099.
Modules, also sometimes known as slabs, are components with a generally square shape and with a thickness of between 25 and 100 mm formed by a plurality of strips held assembled side by side, without compression, each strip consisting of entangled refractory fibers, most of which are perpendicular to the two main faces of the slab. Installation is carried out by adhesion using a cement to the existing linings consisting of bricks or concrete or of a first layer of fiber-based blocks.
Blocks, of parallelepipedal shape, have thicknesses from 100 to 300 mm and are also formed of strips of refractory fibers juxtaposed and precompressed in a casing formed from a plastic film or net. They additionally contain a system, generally metallic, of support for ensuring cohesion of the strips and of attachment to enable them to be quickly put into place.
Among the refractory ceramic fibers used for manufacturing such modules and blocks are AZS fibers, that is to say fibers whose composition mainly consists of alumina, zirconia and silica and which are obtained by melting a mixture of the oxides constituting the refractory composition and dispersion of the molten material as fibers, either by blowing or using one or a number of rotors, as is well known to those skilled in the art. FR-A-1,152,574 and its equivalent U.S. Pat. No. 2,873,197 describe such fibers consisting, by weight, of 40-60% SiO.sub.2, 20-45% Al.sub.2 O.sub.3 and 3.5-20% ZrO.sub.2.
It should be noted that the fibrous masses formed from mixtures of molten oxides generally comprise pseudospherical particles known as "shots" in an amount which can reach 35 to 55% of the total weight. In what follows, however, "fibers" will denote a mass consisting of fibers with a diameter of between 0.5 and 10 .mu.m (on average 1.5 to 3 .mu.m) and of shots, defined by a length/diameter ratio of less than 5. The level of shots is measured according to the NF.40455 test, which makes it possible to estimate the proportion of shots with a size greater than or equal to 40 .mu.m.
However, despite a chemical composition suitable for high temperatures, that is to say an intrinsic refractoriness which is amply sufficient since melting only takes place from 1650.degree. C., it is well known that, in practice, the limit for continuous industrial use of such products based on AZS fibers is of the order of 1330.degree. C.
This is due to transformations which take place at the level of the fibers themselves by a change in their crystallographic composition related to a devitrification phenomenon (transition from the starting amorphous vitreous state to a partially crystalline state at approximately 1000.degree. C. for the AZS fibers) promoted by the alkaline impurities and by the presence of calcium, iron and titanium oxides. This devitrification leads to a significant contraction in size of the individual fibers. The crystallization leads to embrittlement of the fibers which become more rigid and brittle, all the more so as the crystals formed become bigger. The AZS fibers are also subject to sticking at the points of contact between the fibers due to the softening--during the devitrification process--of the residual vitreous phase.
Moreover, a problem specific to modular and block components made of fibers, especially AZS fibers, is that the transformations do not take place throughout the entire volume of the product but progressively, to a certain depth, from the face exposed directly to the heat flow (often known as the hot face by experts), which gradually makes the structure of the whole body heterogeneous.
These phenomena lead to a post-shrinkage at the level of the blocks or modules and cause opening of the joints between the components placed side by side, which is highly prejudicial to the maintenance of the structure of the furnace, because it is then subjected directly to the heat flow and to the corrosive volatile components which prevail in the furnace. In combination with the structural heterogeneity, the post-shrinkage can lead to the fall of the parts which have been transformed to the greatest extent, a phenomenon which is harmful for the usual uses.
There thus exists a need for improved fibrous components made of AZS, such as modular and block components, which can be used continuously at higher temperatures than the current components while retaining the other desirable properties of these components.